Rubber moldings are widely used as seal materials in, for example, chemical equipment, semiconductor producing apparatus, chemical liquid piping and tanks and food producing apparatus.
The rubber moldings are endowed with low friction, nonadherence and abrasion resistance by the following method (1) or (2) in which the rubber material per se is modified or by any of the following methods (3) to (5) in which the surface or vicinity thereof of the rubber material is modified.
In method (1), powder of a resin such as PE, PTFE, POM or a polyester is mixed in the rubber material (solid lubricant adding method).
In method (2), an oil such as PEG, silicone oil or fluorooil is mixed in the rubber material (bleed method).
In method (3), the surface of rubber molding is covered with a resin (surface covering method).
In method (4), the surface of rubber molding is impregnated with a crosslinking agent and heated so that the crosslinking proceeds in the vicinity of the rubber surface (crosslinking agent infiltrating method). In method (5), a monomer which is compatible with the rubber is infiltrated from the surface into the internal part of the rubber molding and polymerized so that the surface vicinity is cured (surface polymerization curing method; see Japanese Patent Laid-open Publication No. 4(1992)-202239).
In the above solid lubricant adding method (1), for example, the solid lubricant mixed in the rubber material hardly comes out on the surface of the rubber molding, so that the functions such as low function and nonadherence cannot be satisfactorily performed. When, in this solid lubricant adding method, a large amount of solid lubricant is added to the rubber base material in order to improve the above functions of the rubber material surface, the mechanical strength and rubber elasticity of the obtained rubber molding are likely to decrease to thereby deteriorate the follow-up to opposite material, and further the compression set thereof tends to increase even if the above functions can be improved.
In the bleed method (2), the mechanical strength of obtained rubber molding is deficient. Further, the bleeding rate of oil mixed in the rubber material is large, and the lubricity of the rubber molding surface varies in conformity with the bleeding rate. After the completion of the bleeding, phenomena such as rapid increase of the friction coefficient of the rubber molding are encountered. Thus, the rubber molding cannot retain stable lubricity for a prolonged period of time. Still further, there is a problem such that oil having bled from the rubber molding stains the opposite material.
In the surface covering method (3), there is the danger that the mutual adherence of the rubber molding and the surface covering layer which constitute the obtained resin covered molding deteriorates, so that the resin covered molding is unsuitable for practical assembly in apparatus as a sealing member for use in dynamic conditions. Further, the rubber elasticity of the resin covered molding surface tends to lower to thereby cause the follow-up to opposite material to deteriorate.
While in the methods (1) to (3) it is intended to perform the functions such as low friction and nonadherence by furnishing the surface of the rubber material with a substance capable of lowering a surface energy of rubber molding by interposition or covering or by blending the substance with the rubber material, it is intended in the crosslinking agent infiltrating method (4) to perform the above functions by crosslinking the vicinity of surface of the rubber material to thereby increase the surface hardness and, hence, reduce primarily the area of contact with opposite surface. However, in the crosslinking agent infiltrating method (4), the composition per se of the rubber molding surface is not different from that of other parts, so that the level of function performance depends on the degree of surface crosslinking. When, thus, the crosslinking of the vicinity of surface (surface layer) of the rubber material is advanced for raising the level of function performance, not only does cracking progress at the surface of the rubber molding but also the deformation follow-up to opposite surface is deteriorated. Therefore, there is the problem that the use of the above rubber molding as a sealing member is likely to cause leakage.
As described in Japanese Patent Laid-open Publication No. 4(1992)-202239, in the surface polymerization curing method (5), the monomers which are infiltrated in the rubber material and polymerized are limited to those having compatibility with the rubber material. Only monomers such as (meth)acrylic acid or a derivative thereof whose molecular weight is relatively small so as to be able to diffuse into rubber network chains can effectively diffuse into the vicinity of surface of the rubber material. The poorer the compatibility between rubber material and monomer (the greater the solubility index difference) and the greater the molecular weight of monomer, the more difficult will it be to infiltrate and diffuse the monomer into the vicinity of surface of the rubber material. Thus, it will also be the more difficult to realize effective performance of the functions such as low friction, nonadherence and abrasion resistance at the surface of the rubber molding.
Accordingly, it is presumed that, if a silicon monomer or a fluoromonomer can be used as the monomer in the surface polymerization curing method (5), the surface energy of the rubber molding would be markedly reduced to thereby be most effective in the performance of the functions such as low friction, nonadherence, abrasion resistance and sealing properties. However, combinations of such a monomer with most rubbers have poor compatibility with each other. For example, combinations of a silicon monomer with HNBR (hydrogenated acrylonitrile-butadiene rubber or nitrile rubber), NBR (acrylonitrile-butadiene rubber, also usually known as nitrile rubber), SBR, (styrene-butadiene rubber), ACM (copolymer of ethyl acrylate or other acrylates and a small amount of a monomer which facilitates vulcanizatio, usually known as acrylic rubber), FKM (fluoro rubber having substituents groups of fluoro, perfluoro, alkyl or perfluoroalkoxy on the polymer chain), CR (chloroprene rubber), CSM (chlorosulfonyl polyethylene), T (rubber having carbon, oxygen and sulfur in the polymer chain, i.e., polysulfide rubber), CO (polychloromethyl-oxirane, usually known as epichlorohydrin rubber), ECO (copolymer of ethylene oxide (oxirane) and chloromethyloxirane, also known as epochlorohydrin copolymer or rubber), ANM (copolymer of ethyl acrylate (or other acrylates) and acrylonitrile) and U (urethane rubber) and combinations of a fluoromonomer with HNBR, NBR, Q (silicone rubber), SBR, ACM, CR, CSM, T, CO, ECO, ANM and U have poor compatibility with each other.
Therefore, in the surface polymerization curing method (5), it is infeasible to infiltrate and diffuse the silicon monomer or fluoromonomer into the vicinity of surface of any of a wide variety of rubber materials and effect a polymerization thereof so as to obtain a rubber molding which is excellent in low friction, nonadherence, abrasion resistance, etc. Moreover, in this surface polymerization curing method (5), when the surface hardness of obtained rubber molding is increased, minute cracks are likely to occur on the surface and the deformation follow-up to opposite surface tends to decrease as in the above crosslinking agent infiltrating method (4). Therefore, there is the danger of deterioration of sealing properties.
As apparent from the above, the method of modifying the rubber material per se, such as the solid lubricant adding method (1) or the bleed method (2), has drawbacks in that it is infeasible to modify the surface or vicinity thereof of rubber material alone and that the properties attributed to the rubber material of the obtained rubber molding, such as mechanical strength, compression set and rubber elasticity, per se are deteriorated by the addition of the solid lubricant, etc. to the rubber material.
On the other hand, the method of modifying the surface or vicinity thereof of rubber material, such as the surface covering method (3), the crosslinking agent infiltrating method (4) or the surface polymerization curing method (5), has drawbacks in that the vicinity of surface of obtained rubber molding is cured to thereby have minute cracks and that the deformation follow-up to opposite surface is deteriorated. Further, the surface covering method (3) has another drawback in that the covering material having properties different from those of the rubber material lacks the compatibility with the inner rubber material, so that it cannot be fixed on the surface of the inner rubber material to thereby cause the adherence of the surface covering layer to the rubber material to be poor.
Japanese Patent No. 2,536,777 (Japanese Patent Laid-open Publication No. 1(1989)-301725) discloses a process for producing a nonadherent fluoroelastomer which comprises the steps of vulcanizing a fluoroelastomer to thereby form double bonds in the fluoroelastomer, infiltrating a reactive silicone resin in the surface of the fluoroelastomer and reacting the infiltrated reactive silicone resin with the fluoroelastomer. It is also disclosed that the infiltration of the reactive silicone resin in the surface of the fluoroelastomer can be effected by dissolving the reactive silicone resin in a solvent such as acetone to thereby obtain a reactive silicone resin solution and immersing the fluoroelastomer in the solution. However, in this process, the rubber which can be treated for surface nonadherence is limited to the fluoroelastomer having double bonds introduced therein, and the type of surface modifying agent usable in the surface treatment is also limited. In this patent (Japanese Patent No. 2,536,777), the reactive silicone resin is used as the surface treating agent. This silicone resin is a polymeric compound, so that, even if the rubber base material is swollen in a solvent as listed in the patent, it is difficult to infiltrate the silicone resin from the rubber surface into the internal part because the molecular weight thereof is too large.
The inventors have made extensive and intensive studies with a view toward solving the problem of how to cause a variety of polymeric compounds whose capability of modifying a rubber surface can be anticipated, inclusive of polymeric compounds set forth as the surface modifier in the above cited patent, to be present in not only the rubber surface but also the rubber surface layer (internal part of the rubber) so as to effectively attain a rubber surface modification. Thus, it has been found that, although the penetration (infiltration) of the polymeric compound per se together with a solvent into the internal part of the rubber as described in the above patent literature involves difficulty, the monomer thereof has a small molecular weight and can be easily infiltrated in the rubber and that, when a polymerizable double bond is present in the monomer, the solubility of the monomer in the polymer generally tends to increase. It has also been found that, when a monomer having a polymerizable double bond which forms a polymeric compound, as a surface modifier, is infiltrated in a rubber swollen by a solvent and polymerized in the rubber, there can be obtained a surface modified rubber which, while retaining the properties such as compression set inherently possessed by the rubber material, is excellent in low friction, nonadherence, abrasion resistance and plasma resistance and can perform the functions attributed to the structure of employed monomer, such as ozone and oil swell resistances. The present invention has been accomplished on the basis of the above findings.
Among rubber moldings composed of various materials, fluoroelastomer moldings are particularly advantageous in that these are excellent in resistances to heat, chemicals and plasma and the polymer per se is stable and in that impurities causing contamination of a semiconductor product or the like, such as an antioxidant generally employed in other rubber materials, are not contained therein so that the fluoroelastomer moldings are excellent in purity of material per se. Therefore, the fluoroelastomer moldings are used as seal materials in liquid crystal/semiconductor producing apparatus, food industry, etc.
However, when used, for example, by being fitted in a moving part of apparatus, the fluoroelastomer molding exhibits the property of tending to stick to opposite material (i.e., sticking tendency or adherence). Thus, the fluoroelastomer molding has a drawback of being inferior in apparatus closing/opening efficiency to other moldings. Moreover, the higher the temperature at which the fluoroelastomer seal material is used, the more conspicuous the sticking tendency. Thus, the conventional fluoroelastomer moldings leave room for improvement in respect of, for example, low friction and nonadherence in the use as a seal material in liquid crystal/semiconductor producing apparatus, food industry, etc. which is often used at ordinary temperature or above. Moreover, in recent years, a plasma generator is used as a semiconductor device. The seal material of plasma treating units represented by a plasma generator is required to have plasma durability so that desired plasma can efficiently be generated in vacuum. The seal material is also required to have a property (low outgassing property) such that gas, etc. is not emitted from the seal material per se so that clean vacuum condition in, for example, a chamber can be maintained.
The present invention has been made with a view toward solving the above problems of the prior art. An object of the present invention is to provide a process for producing a surface modified rubber enabling obtaining a rubber molding which, while retaining the properties such as strength, compression set, sealing capability and deformation follow-up inherently possessed by the rubber material, is excellent in nonadherence, low friction, abrasion resistance and plasma resistance and can perform the functions attributed to the structure of employed monomer, such as ozone and oil resistances.
Another object of the present invention is to provide a surface modified rubber which, while retaining the properties such as strength, compression set, sealing capability and deformation follow-up inherently possessed by the rubber material, is excellent in nonadherence, low friction, abrasion resistance and plasma resistance and can perform the functions attributed to the structure of employed monomer, such as ozone and oil resistances. A further object of the present invention is to provide a seal material comprising the above surface modified rubber.